[PATCH 2/6] KVM: Verify there's at least one online vCPU when iterating over all vCPUs

[Sean Christopherson <seanjc@google.com>]

Explicitly check that there is at least online vCPU before iterating over
all vCPUs.  Because the max index is an unsigned long, passing "0 - 1" in
the online_vcpus==0 case results in xa_for_each_range() using an unlimited
max, i.e. allows it to access vCPU0 when it shouldn't.  This will allow
KVM to safely _erase_ from vcpu_array if the last stages of vCPU creation
fail, i.e. without generating a use-after-free if a different task happens
to be concurrently iterating over all vCPUs.

Note, because xa_for_each_range() is a macro, kvm_for_each_vcpu() subtly
reloads online_vcpus after each iteration, i.e. adding an extra load
doesn't meaningfully impact the total cost of iterating over all vCPUs.
And because online_vcpus is never decremented, there is no risk of a
reload triggering a walk of the entire xarray.

Cc: Will Deacon <will@kernel.org>
Cc: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/kvm_host.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 450dd0444a92..5fe3b0c28fb3 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -985,9 +985,10 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
 	return xa_load(&kvm->vcpu_array, i);
 }
 
-#define kvm_for_each_vcpu(idx, vcpup, kvm)		   \
-	xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \
-			  (atomic_read(&kvm->online_vcpus) - 1))
+#define kvm_for_each_vcpu(idx, vcpup, kvm)				\
+	if (atomic_read(&kvm->online_vcpus))				\
+		xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0,	\
+				  (atomic_read(&kvm->online_vcpus) - 1))
 
 static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 {
-- 
2.47.0.rc0.187.ge670bccf7e-goog